A Strong Lensing Measurement of the Evolution of Mass Structure in Giant Elliptical Galaxies

The structure and evolution of giant elliptical galaxies provide key quantitative tests for the theory of hierarchical galaxy formation in a cold dark matter dominated universe. Strong gravitational lensing provides the only direct means for the measurement of individual elliptical galaxy masses beyond the local universe, but there are currently no large and homogeneous samples of strong lens galaxies at significant cosmological look-back time. Hence, an accurate and unambiguous measurement of the evolution of the mass-density structure of elliptical galaxies has until now been impossible. Using spectroscopic data from the recently initiated Baryon Oscillation Spectroscopic Survey (BOSS) of luminous elliptical galaxies at redshifts from approximately 0.4 to 0.7, we have identified a large sample of high-probability strong gravitational lens candidates at significant cosmological look-back time, based on the detection of emission-line features from more distant galaxies along the same lines of sight as the target ellipticals. We propose to observe 45 of these systems with the ACS-WFC in order to confirm the incidence of lensing and to measure the masses of the lens galaxies. We will complement these lensing mass measurements with stellar velocity dispersions from ground-based follow-up spectroscopy. In combination with similar data from the Sloan Lens ACS (SLACS) Survey at lower redshifts, we will directly measure the cosmic evolution of the ratio between lensing mass and dynamical mass, to reveal the structural explanation for the observed size evolution of elliptical galaxies (at high mass). We will also measure the evolution of the logarithmic mass-density profile of massive ellipticals, which is sensitive to the details of the merging histories through which they are assembled. Finally, we will use our lensing mass-to-light measurements to translate the BOSS galaxy luminosity function into a mass function, and determine its evolution in combination with data from the original Sloan Digital Sky Survey.

ACS/WFC 12210

SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii

Strong gravitational lensing provides the most accurate possible measurement of mass in the central regions of early-type galaxies (ETGs). We propose to continue the highly productive Sloan Lens ACS (SLACS) Survey for strong gravitational lens galaxies by observing a substantial fraction of 135 new ETG gravitational-lens candidates with HST-ACS WFC F814W Snapshot imaging. The proposed target sample has been selected from the seventh and final data release of the Sloan Digital Sky Survey, and is designed to complement the distribution of previously confirmed SLACS lenses in lens-galaxy mass and in the ratio of Einstein radius to optical half-light radius. The observations we propose will lead to a combined SLACS sample covering nearly two decades in mass, with dense mapping of enclosed mass as a function of radius out to the half-light radius and beyond. With this longer mass baseline, we will extend our lensing and dynamical analysis of the mass structure and scaling relations of ETGs to galaxies of significantly lower mass, and directly test for a transition in structural and dark-matter content trends at intermediate galaxy mass. The broader mass coverage will also enable us to make a direct connection to the structure of well-studied nearby ETGs as deduced from dynamical modeling of their line-of-sight velocity distribution fields. Finally, the combined sample will allow a more conclusive test of the current SLACS result that the intrinsic scatter in ETG mass-density structure is not significantly correlated with any other galaxy observables. The final SLACS sample at the conclusion of this program will comprise approximately 130 lenses with known foreground and background redshifts, and is likely to be the largest confirmed sample of strong-lens galaxies for many years to come.

COS/NUV/FUV 12178

Spanning the Reionization History of IGM Helium: a Highly Efficient Spectral Survey of the Far-UV-Brightest Quasars

The reionization of IGM helium likely occurred at redshifts of z=3 to 4. Detailed studies of HeII Ly-alpha absorption toward a handful of quasars at 2.7< z<3.3 confirm the potential of such IGM probes, but the small sample and redshift range limited confidence in cosmological inferences. The requisite unobscured sightlines to high redshift are extremely rare; but we've cross-correlated 10, 000 z>2.8 SDSS DR7 (and other) quasars with GALEX GR4/5, to identify 630 candidates potentially useful for HST HeII studies. Our cycle 15-16 HST trials confirm our approach, verifying twenty new HeII quasars at unprecedented 40% efficiency. We propose to complete the first efficient (80% with refinements) survey for HeII quasars, via reconnaissance (~1 orbit) COS spectra of a highly select subset of 17 SDSS/GALEX quasars at 2.7< z<3.8. Along with past work, this program will yield 3-4 of the brightest far-UV HeII sightlines within each of 10-12 redshift bins spanning 2.7< z<3.8, enabling a community sample suitable for detailed spectral follow-up with HST. Herein, we will also directly obtain quality UV spectral stacks within each redshift bin to trace the reionization history of IGM helium; such spectral stacks average over cosmic variance and individual object pathology. Our high-yield HeII sightline sample and spectral stacks will enable confident conclusions about the IGM baryon density, the spectrum and evolution of the ionizing background, the evolution of HeII opacity, and the epoch of helium reionization.

COS/NUV/FUV 12299

Spectroscopic Signatures of Binary and Recoiling Black Holes

We propose to obtain UV the spectra of the Ly-alpha and Mg II lines of 13 SDSS quasars whose H-beta lines are offset by 1000-4000 km/s from their systemic redshifts. Such lines have been suggested to originate in recoiling or close binary black holes. However these interpretations are not unique and UV spectroscopy, possible only with the HST, can discriminate between competing possibilities. Identifying such systems is extremely important in the context of scenarios for galaxy formation and evolution and in view of recent predictions from numerical relativity. Close binary black holes represent an apparently inevitable stage in the merger of two massive galaxies. The subsequent merger of the members of the binary is expected to produce a recoiling black hole in some fraction of cases. Thus, the census of such systems, their environments, and hosts can constrain some of the more uncertain parameters in evolutionary models. But before we can find them in any numbers, we need to evaluate the candidates known so far. This is the goal of our proposal.

One of the most vexing problems in galaxy formation concerns how gas accretion and feedback influence the evolution of galaxies. In high mass galaxies, numerical simulations predict the initial fuel is accreted through 'cold' streams, after which AGN suppress star formation to leave galaxies red and gas-poor. In the shallow potential wells that host dwarf galaxies, gas accretion can be very efficient, and "superwinds" driven either by hot gas expelled by SNe or momentum imparted by SNe and hot-star radiation are regarded as the likely source(s) of feedback. However, major doubts persist about the physics of gas accretion, and particularly about SN-driven feedback, including their scalings with halo mass and their influence on the evolution of the galaxies. While "superwinds" are visible in X-rays near the point of their departure, they generally drop below detectable surface-brightness limits at ~ 10 kpc. Cold clumps in winds can be detected as blue-shifted absorption against the galaxy's own starlight, but the radial extent of these winds are difficult to constrain, leaving their energy, momentum, and ultimate fate uncertain. Wind prescriptions in hydrodynamical simulations are uncertain and at present are constrained only by indirect observations, e.g. by their influence on the stellar masses of galaxies and IGM metallicity. All these doubts lead to one conclusion: we do not understand gas accretion and feedback because we generally do not observe the infall and winds directly, in the extended gaseous halos of galaxies, when it is happening. To do this effectively, we must harness the power of absorption-line spectroscopy to measure the density, temperature, metallicity, and kinematics of small quantities of diffuse gas in galaxy halos. The most important physical diagnostics lie in the FUV, so this is uniquely a problem for HST and COS. We propose new COS G130M and G160M observations of 41 QSOs that probe the gaseous halos of 44 SDSS dwarf galaxies well inside their virial radii. Using sensitive absorption-line measurements of the multiphase gas diagnostics Lya, CII/IV, Si II/III/IV, and other species, supplemented by optical data from SDSS and Keck, we will map the halos of galaxies with L = 0.02 - 0.3 L*, stellar masses M* = 10^(8-10) Msun, over impact parameter from 15 - 150 kpc. These observations will directly constrain the content and kinematics of accreting and outflowing material, provide a concrete target for simulations to hit, and statistically test proposed galactic superwind models. These observations will also inform the study of galaxies at high z, where the shallow halo potentials that host dwarf galaxies today were the norm. These observations are low-risk and routine for COS, easily schedulable, and promise a major advance in our understanding of how dwarf galaxies came to be.

STIS/CCD 11845

CCD Dark Monitor Part 2

Monitor the darks for the STIS CCD.

STIS/CCD 11847

CCD Bias Monitor-Part 2

Monitor the bias in the 1x1, 1x2, 2x1, and 2x2 bin settings at gain=1, and 1x1 at gain = 4, to build up high-S/N superbiases and track the evolution of hot columns.

We will use the unique power of WFC3 slitless spectroscopy to measure cosmic star formation across its peak epoch. The broad, continuous, spectral coverage of the G102 and G141 grisms provides the best currently feasible measurement of the star formation rate continuously from 0.5< z<2.5, over which ground-based searches are severely limited. Our Cycle 17 pure-parallel grism program has proven efficient for identifying line emission from galaxies across this large fraction of cosmic time. With less than two months of WFC3 observing completed, our new measurements have more than doubled the sample of emission-line galaxies that we found over the entire NICMOS Parallel Grism program. We propose to extend this cost-effective WFC3 Survey by using 280 orbits of pure parallel grism spectroscopy in 50 deep (4-5 orbit) fields with both G102 and G141, and 40 shallow (2-3 orbit) fields with G141 alone. This will complete a sample of 2000-3000 emission line galaxies in the "redshift desert" and search for serendipitous Lya emitters at z>5.5.

Our primary science goals are: (1) Measure ratios of bright emission lines ([OII], [OIII], Ha, and Hb) in a substantial fraction of these galaxies, thereby estimating dust and metallicity evolution in a sample of galaxies that is not biased by photometric selection. (2) Derive an extinction-corrected Ha luminosity function, with a 20 times larger sample than our previous NICMOS results. (3) Measure the mass-metallicity relation at crucial intermediate redshifts, with the support of our ongoing ground-based, follow-up, observing program (4) Determine the spectroscopic close pair fraction in this sample, in order to constrain hierarchal merging models (5) Uncover a new sample of obscured AGN at these redshifts and, (6) Use the Balmer break diagnostic to constrain the ages of continuum detected sources down to H = 25.

As a bonus, these observations will be sensitive to Lya emission at z>5.5, taking advantage of continuous spectral coverage to observe large volumes for luminous galaxies at the highest redshifts. Over Cycles 17 and 18, we expect to detect 5-20 LAEs over redshifts spanning 5.5 < z < 7.5. These observations will likely place the most stringent constraint on the numbers of z>6.5 Lya emitters until JWST. We are waiving all proprietary rights to our data and will make high-level data products available through the ST/ECF.

WFC3/IR/S/C 11929

IR Dark Current Monitor

Analyses of ground test data showed that dark current signals are more reliably removed from science data using darks taken with the same exposure sequences as the science data, than with a single dark current image scaled by desired exposure time. Therefore, dark current images must be collected using all sample sequences that will be used in science observations. These observations will be used to monitor changes in the dark current of the WFC3-IR channel on a day-to-day basis, and to build calibration dark current ramps for each of the sample sequences to be used by Gos in Cycle 17. For each sample sequence/array size combination, a median ramp will be created and delivered to the calibration database system (CDBS).

WFC3/IR/UV 12163

Structure and Stellar Content of the Nearest Nuclear Clusters in Late-Type Spiral Galaxies

HST surveys have shown that nuclear star clusters are nearly ubiquitous in late-type, bulgeless disk galaxies. In early-type galaxies, the central black hole mass correlates with the bulge mass and velocity dispersion, but the relationship between black hole mass and host galaxy properties in bulgeless galaxies is not yet understood. Some nuclear clusters (such as the one in M33) do not contain a central massive black hole at all, while other late-type galaxies (such as NGC 4395) are known to contain accretion-powered active nuclei within their nuclear clusters, indicating that a central black hole is present. But, the overall "occupation fraction" of black holes within nuclear clusters is largely unconstrained. Measurement of the structure, dynamics, and stellar content of nuclear star clusters is an important pathway toward understanding the demographics of low-mass black holes in late-type galaxies.

We propose to obtain multi-filter WFC3 UV, optical, and near-IR images of 10 of the nearest and brightest nuclear clusters in late-type spiral galaxies. We will use the new WFC3 data to measure the cluster radial profiles, to search for color gradients, and in combination with ground-based spectroscopy and stellar population modeling, to determine the stellar masses of the clusters. Since nuclear clusters are known to contain stellar populations with a wide range of ages, the broad wavelength coverage of our data will provide new leverage to constrain the star formation history of the clusters. We will carry out dynamical modeling for the clusters, using the cluster structural parameters and stellar M/L ratios measured from the WFC3 data and kinematics measured from ground-based, adaptive-optics assisted integral-field spectroscopy (already obtained or approved for 8 of the 10 targets). This will yield tight new constraints on the masses of intermediate-mass black holes (IMBH) within the clusters, and may result in the first dynamical detections of IMBHs in the nuclei of late-type spirals.

WFC3/UV 12215

Searching for the Missing Low-Mass Companions of Massive Stars

Recent results on binary companions of massive O stars appear to indicate that the distribution of secondary masses is truncated at low masses. It thus mimics the distribution of companions of G dwarfs and also the Initial Mass Function (IMF), except that it is shifted upward by a factor of 20 in mass. These results, if correct, provide a distribution of mass ratios that hints at a strong constraint on the star-formation process. However, this intriguing result is derived from a complex simulation of data which suffer from observational incompleteness at the low-mass end.

We propose a snapshot survey to test this result in a very direct way. HST WFC3 images of a sample of the nearest Cepheids (which were formerly B stars of ~5 Msun) will search for low-mass companions down to M dwarfs. We will confirm any companions as young stars, and thus true physical companions, through follow-up Chandra X-ray images. Our survey will show clearly whether the companion mass distribution is truncated at low masses, but at a mass much higher than that of the IMF or G dwarfs.

WFC3/UV 12245

Orbital Evolution and Stability of the Inner Uranian Moons

Nine densely-packed inner moons of Uranus show signs of chaos and orbital instability over a variety of time scales. Many moons show measureable orbital changes within a decade or less. Long-term integrations predict that some moons could collide in less than one million years. One faint ring embedded in the system may, in fact, be the debris left behind from an earlier such collision. Meanwhile, the nearby moon Mab falls well outside the influence of the others but nevertheless shows rapid, as yet unexplained, changes in its orbit. It is embedded within a dust ring that also shows surprising variability. A highly optimized series of observations with WFC3 over the next three cycles will address some of the fundamental open questions about this dynamically active system: Do the orbits truly show evidence of chaos? If so, over what time scales? What can we say about the masses of the moons involved? What is the nature of the variations in Mab's orbit? Is Mab's motion predictable or random? Astrometry will enable us to derive the orbital elements of these moons with 10-km precision. This will be sufficient to study the year-by-year changes and, combined with other data from 2003-2007, the decadal evolution of the orbits. The pairing of precise astrometry with numerical integrations will enable us to derive new dynamical constraints on the masses of these moons. Mass is the fundamental unknown quantity currently limiting our ability to reproduce the interactions within this system. This program will also capitalize upon our best opportunity for nearly 40 years to study the unexplained variations in Uranus's faint outer rings.

WFC3/UV 12324

The Temperature Profiles of Quasar Accretion Disks

We can now routinely measure the size of quasar accretion disks using gravitational microlensing of lensed quasars. At optical wavelengths we observe a size and scaling with black hole mass roughly consistent with thin disk theory but the sizes are larger than expected from the observed optical fluxes. One solution would be to use a flatter temperature profile, which we can study by measuring the wavelength dependence of the disk size over the largest possible wavelength baseline. Thus, to understand the size discrepancy and to probe closer to the inner edge of the disk we need to extend our measurements to UV wavelengths, and this can only be done with HST. For example, in the UV we should see significant changes in the optical/UV size ratio with black hole mass. We propose monitoring 5 lenses spanning a broad range of black hole masses with well-sampled ground based light curves, optical disk size measurements and known GALEX UV fluxes during Cycles 17 and 18 to expand from our current sample of two lenses. We would obtain 5 observations of each target in each Cycle, similar to our successful strategy for the first two targets.

WFC3/UV 12345

UVIS Long Darks Test

Darks during SMOV showed a systematically lower global dark rate as well as lower scatter when compared to the Cycle 17 darks. Those two sets of exposures differ in exposure time - 1800 sec during SMOV and 900 sec during Cycle 17. Hypothetically, the effect could be caused by short-duration stray light, say ~500-sec in duration. During the latter part of Cycle 17, operation of WFC3 was changed to additionally block the light path to the detector with the CSM. This program acquires a small number of darks at the longer SMOV exposure times (1800 sec) in order to check whether the effect repeats in the new operating mode.

WFC3/UVIS 11905

WFC3 UVIS CCD Daily Monitor

The behavior of the WFC3 UVIS CCD will be monitored daily with a set of full-frame, four-amp bias and dark frames. A smaller set of 2Kx4K subarray biases are acquired at less frequent intervals throughout the cycle to support subarray science observations. The internals from this proposal, along with those from the anneal procedure (Proposal 11909), will be used to generate the necessary superbias and superdark reference files for the calibration pipeline (CDBS).